1. Field of the Invention
The present invention relates to a method for remedying polluted soil. In more detail, this invention relates to a method for remedying polluted soil in which the pollutant in the polluted region is decomposed by microbial activities.
2. Related Background Art
Recent rapid developments in science and technology have produced a vast amount of chemicals and chemical products. These substances are polluting nature slowly accumulating in the environment. Environmental pollution is a serious problem spreading all over the world since water and the air are circulating in the environment. Examples of hitherto known pollutants are chlorinated organic compounds (such as dichloroethylene (DCE), trichloroethylene (TCE), tetrachloroethylene (PCE) and dioxin), aromatic compounds (such as toluene, xylene and benzene) and fuels such as gasoline. Chlorinated aliphatic hydrocarbon compounds (such as dichloroethylene, trichloroethylene and tetrachloroethylene) are especially used in a large amount as a solvent for cleaning precision machine members and for dry cleaning, and pollution of soil and ground water by these solvents have been revealed. In addition, these organic compounds are so volatile that they may cause air pollution. It is also pointed out that these organic compounds are teratogenic and carcinogenic, so that it becomes evident that they seriously affect living creatures. Accordingly, an urgent theme is not only to cut off the pollution sources but also to clean the soil and ground water already polluted with these organic compounds.
One of the conventional methods for remedying the soil polluted with chlorinated organic compounds is, for example, to scoop out the polluted soil and subject it to a heat treatment. Although this method enables complete elimination of pollutants from the dug up soil, it requires much expenses and a long working period for turning up the soil. It is practically impossible to take out the polluted soil situated deep under the ground, limiting the application range of this method. In addition, the chlorinated organic compounds released from the dug up polluted soil should be recovered by adsorption onto an adsorbent such as activated charcoal to prevent secondary air pollution, and the used activated charcoal requires further processing. For example, when the used activated charcoal which adsorbed chlorinated compounds such as DCE, TCE and PCE is incinerated, more poisonous by- products such as phosgene may be generated. Accordingly, the final processing cost is predicted to be enormous because of the necessary additional steps to make the recovered pollutants harmless.
Vacuum-extraction of the pollutants from the polluted soil or use of a microorganism having pollutant-degrading ability can solve one of the problem of the above-mentioned method, i.e., limitations of the treating region. These methods do not require to dig up the soil and can purify the soil at the location where it is (called xe2x80x9cin situxe2x80x9d hereinafter). Actually, these method are cheap and simple compared with the foregoing dig-up method; only small-scale work is required such as boring a well for introducing a vacuum extraction pipe or pollutant-degrading microorganisms into the polluted soil. The vacuum extraction method has problems that it cannot remove chlorinated organic compounds in a low concentration of several ppm or less efficiently, and that further treatment of the recovered chlorinated organic compounds is required as in the above-mentioned method.
On the other hand, the pollutant in soil can be degraded into harmless substance(s) by the microbial remediation method using microorganisms native or foreign to the soil. Thus, the microbial method dispenses the detoxification treatment of the recovered pollutant that is indispensable in the foregoing two methods. In addition, this method is highly efficient in degrading pollutant of a relatively low concentration.
Accordingly, now the microbial remediation method is attracting attentions.
When the native microorganisms (inherently living in the region to be remedied) are used in the remediation method, it is necessary to supply the soil region to be treated with activating agents such as inducers to induce degradation activity of the native microorganisms, nutrients to enhance the microbial degradation activity, oxygen and growth stimulating agents. When a foreign microorganism having the pollutant-degrading ability is used, it is necessary to introduce into the soil the microorganism and if necessary activating agents for that microorganism.
In both cases, it is preferable to introduce the microorganism or the activating agent in the soil as even as possible. Usually, soil structure is not so uniform as to allow uniform diffusion of a liquid containing the microorganism and activating agent into the soil. For the purpose of solving these technical problems, the inventors of the present invention have disclosed an art for uniform distribution in the soil of the injected liquid containing a microorganism and a microbial activating agent. Japanese Laid-Open Patent Application No. 8-224566.
The inventors of the present invention has found that when a liquid containing a microorganism and an activation agent is injected into the soil, a portion of the pollutant present in the voids (pores) of the soil may be pushed out according to the injection, and move along the diffusion of the liquid, so that the polluted region may be expanded by the liquid injection. This tendency is more evident with volatile pollutants such as DCE, TCE and PCE. Therefore, enlargement of the polluted region due to the liquid injection should be prevented as much as possible irrespective of the pollutant concentrations, especially in In situ remediation of the soil. As a conclusion, a technical development has been required to solve this problem.
Further studying how to solve the technical problems hitherto described, the inventors of the present invention found a method for remedying the soil which completes remediation of the soil in the closed space by isolating the polluted region in situ from the surrounding soil, or substantially enclosing the soil within a closed space.
The object of the present invention, based on the findings of the inventors of the present invention, is to provide a method for carrying out high remediation of the soil while preventing enlargement of the polluted region.
In accordance with one embodiment of the present invention, there is provided a method for remedying soil containing a region polluted with a pollutant which comprises a step of injecting into the soil a liquid containing a microorganism having an activity to decompose the pollutant or a liquid containing a microorganism having ability to decompose the pollutant and an activation agent for the microorganism, wherein the step comprises:
isolating the region from surrounding soil with a barrier made of a material that does not allow the pollutant, the microorganism, the activation agent or water to pass through; and
replacing void water in the isolated region with said liquid.
In accordance with another embodiment to achieve the foregoing object, the present invention provides a method for remedying soil containing a region polluted with a pollutant comprising a step of injecting into the soil a liquid containing a microorganism having an activity to decompose the pollutant or a liquid containing a microorganism having ability to decompose the pollutant and an activation agent for the microorganism, wherein the step comprises:
separating the region from the surrounding soil with a barrier made of a material that does not allow the pollutant, the microorganism, the activation agent or water to pass through; and
injecting into the region the liquid in an amount 1.1 times or more a volume of the void of the isolated region.
In accordance with the other embodiment, the present invention provides a method for remedying a soil polluted with a pollutant comprising a step of taking the soil in a treatment vessel to decompose the pollutant by introducing a liquid containing a microorganism capable of decomposing the pollutant or a liquid containing the microorganism and an activation agent for the microorganism, wherein the treatment vessel is composed of a material that does not allow the pollutant, microorganism, the activation agent or water to pass through, and the liquid is injected to replace void water in the soil in the vessel with the liquid.